zdtm: test -- Add trivial poll() timeout test

The idea is simply make sure that timeour has not been
screwed during c/r cycle.

v2:
  - Drop unneeded early poll() call.
  - Make poll loop in cycle until caller passes c/r cycle.
v3:
  - Use test_go helper
  - Do poll in cycle

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>